.body {
    height: 100%;
    display: grid;
    grid-template-rows: auto 1fr;
    grid-template-areas: 
        "h"
        "m";
}

.header {
    grid-area: h;
}

.main {
    grid-area: m;
}

.iframe {
    display: block;
    width: 100%;
    height: 100%;
}